HIV-1 strains are highly variable and this diversity provides a major challenge for vaccine design. A candidate vaccine should provide protection against most clades of HIV. To address this problem, approaches to maximizing immunological strength and breadth are being explored, including strategies that use consensus, center-of-tree or ancestral sequences, multiple strains or mosaic immunogens, immunogens consisting of known epitopes from the database, and chimeric, molecules expressing a selection of the most conserved epitopes from different clades of HIV [1-17, 56, 96 ].